If it's not open it's broken.

Samsung Locks Down Carrier-Branded Phones with Latest OTA Update

If you’re financing your ultra-premium Samsung flagship through your carrier, as most folks in North America do, it’s entirely understandable that you’d want to de-bloat your phone—see this post from 2013 for some examples of Canadian carrier bloat on a Galaxy S4. One of the easier ways to do this has been to flash a different firmware onto your device; though Samsung phones are region-locked it’s been possible to flash an in-region but non-carrier version of your device’s firmware using tools and guides from XDA.

Until now, that is: XDA reported yesterday that the January security update for the S8, S8+ and Note8 also includes a new bootloader, one that prevents the flashing of unlocked firmware on carrier-branded phones. If you try to change the firmware on your carrier-branded device you will hard-brick that device. Unlocked hardware purchased from Samsung or third parties seems unaffected.

Though not explicitly stated by anyone on XDA or the cross-post to r/Android, it sounds to me like this “update” would also prevent users from flashing custom ROMs onto late model carrier-branded Samsungs.